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The Journal of the American Medical Associationhas published a rigorous comparison of several popular exercise trackers:

With the exception of the Nike Fuelband most of the trackers did fairly good job of measuring steps. However, the Digi-walker SW-200 , a less expensive dedicated pedometer, out performed many of the smartphone apps.

I have been very happy with my Omron pedometer. For me, the only drawback is the inability to sync it to statistical software.

But overall, the message seems to be that most exercise trackers will give you a reasonable approximation of how much you exercise.

For more discussion of these findings see this interesting post by Ernesto Ramirez at The Quantified Self.